PTPIP51 in protein interactions: regulation and in situ interacting partners.

This study investigated the regulation of 14-3-3β binding to PTPIP51 by the tyrosine phosphorylation status of PTPIP51. The tyrosine 176 residue is phosphorylated by c-Src. Up to now, nothing is known about the impact of such well-established phosphorylation events on the interaction profile of PTPIP51 with its partners of the mitogen-activated protein kinase (MAPK) pathway. In human keratinocytes the PTPIP51 phosphorylation was varied by inhibiting the phosphatase activity, thus enhancing the phosphorylation of PTPIP51. Differential blocking of Src kinase family members (despite c-Src) by PP2 increased the activity of c-Src and the tyrosine phosphorylation of PTPIP51 at position 176, which is the substrate of c-Src kinase. The amount of PTPIP51 interactions with 14-3-3β, Raf-1, PTP1B and c-Src was evaluated and the resulting data were compared to an untreated control group. The increased phosphorylation level resulted in a sharp drop of the 14-3-3β/PTPIP51 and 14-3-3β/Raf-1 interaction. Besides the 14-3-3 interaction of PTPIP51, the interaction with the two MAPK modulators, protein kinase A (PKA) and diacylglycerol kinase alpha (DAGKα), are also regulated by the tyrosine phosphorylation status of PTPIP51. Additional immunostaining experiments were done investigating the functional implication on these interactions of the phosphorylation in apoptotic processes. In the pervanadate- and PP2-treated HaCaT cells, higher amounts of apoptotic cells were not detected as compared to the control group. The presented data confirms a tyrosine phosphorylation-dependent interaction of PTPIP51 with 14-3-3β and Raf-1 in vivo and a tyrosine-dependent interaction profile with DAGKα and PKA. The non-interaction of PTPIP51 with 14-3-3 is not sufficient for triggering apoptosis.